Who is the man in the credit scene in The Amazing Spider-Man?

Well, according to this page on The Amazing Spider Man Wikia, and Entertainment Weekly, this is Gustav Fiers, AKA The Gentleman, a sinister figure with ties to Oscorp and the Sinister Six. From EW (about a scene in the 2nd movie):

The final moments also feature a scene with Harry, now in his own cell in Ravencroft, taking a call from The Gentleman otherwise known as Mr. Fiers (whom we all remember from his appearance at the end of the first film.) Harry is looking better — we can assume that the Goblin suit equalized his condition enough so that he could be functional — and he talks about putting together a team to beat Spider-Man. Important phrasing in this short scene: “I want to keep it small.” Without question, this is our introduction to the Sinister Six, as we then see The Gentleman start to peruse the goods in the special lab section of Oscorp (key among them: Doc Ock’s arms, Vulture’s wings, and the Rhino suit.)

Here's the link. MTV (scroll to the bottom) and The Toronto Sun (4th paragraph from the end) also seem to treat it as fact.